Who is Arthur Koestler, and Should we Care?

The amount of information which is at one's fingertips is mind boggling; and the more one delves and clicks on links, the further one gets from the original point of departure. One such virtual adventure happened to me as I was researching the differences and similarities between Sephardic and Ashkenazi Jews, a topic which fascinates me.
Arthur Koestler's name popped up, linked with a study he made on the kingdom of Khazar. That was a word I had heard before, and knew very little about it actually.He called them the "Thirteenth Tribe" going back into history to the 6th century, and converting to Judaism. He goes on to say that the modern day Ashkenazi Jew is a direct descendant of these Khazars, and formed the base of Polish Jews in Eastern Europe.
It would then appear that the Jews of Palestine who lived there for millenia were never of European origin, but were those to whom the label "Sephardic" applies. The Ashkenazi Jews greatly outnumber the Sephardic Jews, and are the majority rule in Israel.
When Koestler's findings were published almost 40 years ago, one could imagine the tremors it must have sent through the Ashkenazi Jewish communities of the world. Coincidentally, as the controversy reached its peak in 1983, both Koestler and his wife were found dead in their London apartment. Suicide was ruled out. What do you think?
Mr. Koestler was an Ashkenazi Jew and took pride in his Khazar ancestry. He was also a very talented and successful writer who published over 25 novels and essays. His most successful book, Darkness at Noon, was translated in thirty-three languages.